Thyroxine-binding globulin (TBG) has been deglycosylated with a mixture of glycosidases. The results indicate that the oligosaccharides of TBG, which contains 23% carbohydrate, are not involved in thyroid hormone binding or in the antigenicity of the protein. Electron spin resonance was used to study the topography of T4-binding sites in the serum transport proteins by employing a series of "spin"-labeled thyroxine analogs. The binding site of serum albumin is deeper than that of either prealbumin or TBG. T4 is less rigidly held in the TBG site despite a much higher binding constant.